The invention relates to a method and an apparatus for producing a punching tool, especially for label cutting apparatus, which has a sharp cutting edge, wherein a punch body is provided in a first process step with a raised edge and this edge is sharpened in a second process step to form sharp cutting edge.
For cutting through soft materials, such as papers, plastic films, textiles, flexible circuit boards, labels or the like, punching devices are mostly used which have punches with sharp cutting edges. The punches can consist, for example, of platens, cylinders, or flexible plates held on cylinders, the cutting edges of which project in the form of sharpened, raised edges.
The cutting edges should have a uniform height over the entire punch. In the manufacture and further processing of pressure-sensitive or gummed labels, for example, the starting material consists of a label paper provided with an adhesive coating. The purpose of the punch is to act on the starting material such that the label paper will be completely cut through according to the desired contour of the labels, but the backing strip will remain virtually uncut. If this requirement is not satisfied trouble will be encountered in the automatic production of the labels whenever the "lattice" surrounding the labels is removed from the backing strip or when, during the further handling of the labels in an automated production apparatus, such as a packaging machine or an envelope addressing and labeling machine, the labels themselves are pulled from the backing strip to be applied to an object. If the label paper has not been cut through cleanly all around in the punching process, the "lattice" pulls the labels with it, resulting in a nuisance and in some cases a long shut-down of the entire production system Similar disadvantages result when not only the label paper but also the backing is wholly or partially cut through, since in this case the labels cannot be released correctly without pulling pieces of the backing with them. Since the label paper often has a thickness of only 0.1 mm and the backing a thickness of only 0.05 mm, the height of the cutting edges of the punch must satisfy tolerances of several hundredths of a millimeter down to less than one hundredth of a millimeter. Such high tolerances are desired also when paper webs without a protective backing web are involved and, for example, a window is to be punched into a mailing envelope so that the backing used in the punching process and consisting for example of rubber or steel will neither be destroyed nor cause the rapid dulling of the cutting edges.
It has been common practice heretofore to form the raised edges in a first process step, for example by removing the areas surrounding them by erosion or etching and then, in a second process step, to provide the raised edges with sufficiently sharp cutting edges by manual, mechanical treatment, because with the etching methods known heretofore neither the desired thin cutting edges of about 0.01 mm and less nor the necessarily close height tolerances can be guaranteed. The method used in the second step of the process consists mostly in scraping the raised edges with a sharp edge of an engraving burin or the like, manually, from at least one side under a magnifying glass, which not only is a time-consuming, tiring activity and calls for well-trained workers, but also easily leads to flaws which can result in the rejection of an entire punching die. Working from the sides furthermore requires that the original punch body from which the punch is made be itself subject to close tolerances, so that only high-quality materials may be used for this purpose. Overall, this results in slow deliveries of the punch bodies and/or in slow production of the punches, which is intolerable to the people ordering and using the punches, who must retool quickly and at reasonable prices to produce the label formats or the like which their customers desire.
The invention is addressed to the task of devising a method and an apparatus by means of which the manual finishing of a raised edge produced by etching or otherwise on a punch can be performed semiautomatically. The invention furthermore is to make it possible for the height tolerances in the production of the raised edges in the first step of the process to be less critical than heretofore, and to some extent to equalize variations in thickness of the punch bodies.